Investigation of drill-in fluids damage and its impact on wellbore stability in Longmaxi shale reservoir

The stability of the horizontal long section wellbore is the key factor of restricting the efficient development of shale gas. High clay mineral content and developed microfractures of shale reservoir make the problems of formation damage and horizontal wellbore stability more complex. OBDF (Oil-based drill-in fluid) is widely used to control formation damage and prevent wellbore instability in shale gas wells. However, high cost and serious environmental problem severely restrict the economic development of shale gas. In order to meet the requirement of cost reduction and environmental protection during shale gas drilling operation, drilling horizontal well with WBDF (water-based drill-in fluid) instead of OBDF has become one of the engineering technology problems to be solved urgently. The Longmaxi shale in Sichuan Basin of China and the associated drill-in fluids at the site are the object of our research. Evaluation experiments on liquid damage and sealing capacity of drill-in fluids were conducted to recognize and optimize the properties of oil-based and water-based drill-in fluids in shale reservoir. Combining the methods of particle size distribution, friction coefficient testing, alkali erosion, linear expansion and immersion test, we investigated the effect of drill-in fluids on formation damage and wellbore stability. Results show that the damage degree of OBDF filtrate on shale is more severe than that of WBDF filtrate. Fracture sealing and bi-directional pressure containment capacity of WBDF is also stronger than that of OBDF. Due to the invasion of OBDF with high pH value, the friction strength of the fracture surface decreases and frictional sliding will take place, which can cause the wellbore instability. With the WBDF immersion, microfractures initiate and propagate along the bedding surface for the shale hydrate expansion, and lead to wellbore instability. The results of our analysis suggest that OBDF does not completely solve the problems of formation damage and wellbore instability in shale reservoir, and reinforcing the sealing fracture and inhibiting hydration capacity of WBDF will contribute to its wider application in shale reservoir. This study provides a theoretical guidance for drill-in fluid selection and optimization during the operation of shale gas well drilling.